So you pay tax once RSU vest even if you have not sold it. It kind of sucks, so all the lyft people have to pay like 70 per stock income even though they are like way down if they vested on first day of IPO.

Brokerages are required by law to withhold tax on vests. By default they do this by selling some of the shares immediately that vest. So you think you are getting 100 but you actually end up with 70 or whatever. It's better than a big bill on tax day though.